WASHINGTON — The fresh evidence on Friday of the economy’s downward spiral focused even more attention on two questions: Is the stimulus package being pushed by President-elect Barack Obama big enough? And will the component parts being assembled by Congress provide the most bang for the buck?

With the Federal Reserve having just about reached the limit of how much it can help the economy with cuts in the interest rate, Washington’s ability to end or at least limit the recession depends in large part on the effectiveness of the big package of additional spending and tax cuts that Mr. Obama has made the centerpiece of his agenda.

And with the economy facing what now seems sure to be the sharpest downturn since the 1930s, the financial system balky and the government facing towering budget deficits, economists and policy makers acknowledge that there is no playbook.

“We have very few good examples to guide us,” said William G. Gale, a senior fellow at the Brookings Institution, the liberal-leaning research organization. “I don’t know of any convincing evidence that what has been proposed is going to be enough.”

In part because Mr. Obama wants and needs bipartisan support, the package is being shaped by political as well as economic imperatives, complicating the process by putting competing ideological approaches into the mix.

It includes $300 billion in temporary tax cuts for individuals and businesses, in part to attract Republican support. It includes a big expansion of safety-net programs like unemployment insurance, which Democrats say makes both economic and social sense. It includes more money for highways, schools and other public infrastructure; more money for “green” energy projects; and more money to help state governments pay for health care and education.

Republicans, as always, are advocating for more and broader tax cuts. But the evidence is ambiguous about whether tax cuts will really spur economic activity at a time when consumers and businesses alike are frozen in fear and reluctant to let go of their money.

The risk is that Mr. Obama and the Congress will weigh down their effort with measures that cost many billions of dollars but may not have much impact on economic activity.

Tax breaks, for example, usually produce less than $1 of stimulus for every dollar they cost, economists say. Spending on public construction projects, like highways and bridges, produces the most economic activity — but there is a limit to how many projects are “shovel-ready,” and even those take time to generate jobs and ripple through the economy.

Christina Romer, whom Mr. Obama has designated to be his chief economist, concluded in research she helped write in 1994 that interest-rate policy is the most powerful force in economic recoveries and that fiscal stimulus generally acts too slowly to be of much help in pulling the economy out of recessions, though associates said she now supports a big stimulus package if policy makers roll it out early enough in the recession.

The goal behind all those ideas is to jump-start economic activity by getting as much money as possible as quickly as possible into the hands of consumers and businesses, trying to make up for the falling demand in the private sector that is leading to higher unemployment. And although the package includes a big dose of tax cuts, it represents a big departure from President Bush’s playbook by relying heavily on direct government spending.

“This is not an intellectual exercise, and there’s no pride of authorship,” Mr. Obama told a news conference in Washington on Friday. “If members of Congress have good ideas, if they can identify a project for me that will create jobs in an efficient way — that does not hamper our ability to, over the long term, get control of our deficit; that is good for the economy — then I’m going to accept it.”

Mr. Obama’s aides said he did not intend to unveil a detailed formal proposal but rather to allow Congress to fill in the outline that he has proposed.

Given the recent scale of the downturn — the nation lost 1.5 million jobs in the last three months of 2008, and economic output during those months shrank by 6 percent compared with same period in 2007 — economists were highly uncertain about whether the economic plan would provide enough firepower.